Calcium halophosphate phosphors have had wide application as fluorescent lamp phosphors. These phosphors are typically activated with antimony and manganese and may be adjusted in composition to provide white light, upon excitation by ultraviolet radiation, which varies from "cool" to "warm" white. Typical phosphors are "Cool White", Sylvania.TM. type 4450 and "Warm White", Sylvania.TM. type 4300. It is known in the art that the quantum efficiency of these phosphors can be affected by increasing the concentration of the antimony activator. For example, U.S. printed patent application B 288,018, issued to Wanmaker et al., describes a method of manufacturing halophosphate phosphors having higher antimony concentrations by contacting the reaction mixture with gaseous antimony trioxide generated from various antimonates. Other methods for making these types of phosphors are disclosed in U.S. Pat. Nos. 2,965,786, 3,379,649, 3,798,479, 3,255,373, and 5,232,626.
The present invention is an improvement over the prior art because it provides calcium halophosphate phosphors having higher quantum efficiencies at specific antimony concentrations by reducing the phosphor's particle size. Thus, the quantum efficiencies of calcium halophosphate phosphors may be increased without having to increase the levels of the antimony activator.
Generally, phosphors in fluorescent lamp applications convert the ultraviolet light emitted by the mercury discharge into visible light. For example, if an ultraviolet (UV) photon of 2540 .ANG. is absorbed by the phosphor, then the phosphor might emit a lower energy photon of 5000 .ANG.. The specific energy processes involved will depend on the exact composition of the phosphor.
Absorption of energy by a phosphor may occur in the host lattice or directly in the activator site. The activator site absorbs energy and changes its electronic state from a ground state to an excited state. Although several excited states are generally possible, only the lowest energy excited state is involved in photon emission. The proper choice of host and activator is essential to obtain an efficient phosphor. During excitation of the phosphor, some of the incident photons are reflected, some are transmitted, and, if the phosphor is an efficient combination of host and activator, most are absorbed. However, not all of the absorbed photons, or quanta, result in excitation of the activator, nor do all of the excited activator sites result in the emission of a photon. Some of the centers become deactivated by radiationless transitions.
In general, the energy (E) dissipation process in a phosphor can be represented as: EQU E.sub.absorbed =E.sub.original -E.sub.reflected EQU E.sub.excitation =E.sub.absorbed -E.sub.lattice absorption
The total energy balance can be expressed as: EQU E.sub.original =E.sub.emission +E.sub.excitation +E.sub.lattice absorption +E.sub.reflected
where E.sub.original is the total incident energy on the phosphor.
The quantum efficiency (QE) of a phosphor can be expressed as: ##EQU1## While little can be done to change the excitation energy, E.sub.excitation, and the host lattice absorption, E.sub.lattice absorption, because they are intrinsic to the phosphor composition, the energy reflected, E.sub.reflected, can be altered by changing the morphology of the material. For an efficient phosphor at its maximum E.sub.emission, (emitted energy), an increase in E.sub.reflected will only lead to a small change in E.sub.emission. If the decrease in E.sub.emission is smaller than the increase in E.sub.reflected, an increase in QE will be observed yielding a more efficient phosphor.